Free Cartilages of the Zebrafish Danio rerio (Danionidae) Caudal Fin Endoskeleton

The caudal fin endoskeleton of many actinopterygian taxa contains free cartilages at the base of the lepidotrichia of the main and procurrent rays. Even in popular model fish species, such as the zebrafish Danio rerio (Danionidae), these structures are often overlooked in studies of fish anatomy and development since they appear late in ontogeny, are hidden by the bases of the lepidotrichia, and are usually not ossified. We revealed distal cartilage of preural 2 vertebra, ventral and dorsal caudal radials in the caudal fin of D. rerio with varying frequency of occurrence. Distal cartilage of preural 2 vertebra and ventral caudal radial are characteristics of the D. rerio and are deeply integrated into the morphofunctional system of the caudal complex. Dorsal caudal radial rarely occurred in the studied samples, and can be regarded as atavism.